This invention relates generally to high-frequency active devices, such as Gunn effect oscillators and amplifiers, and, more particularly, to such devices operating in the microwave and millimeter-wave range.
The Gunn effect was first pointed out by J. B. Gunn in an article published in the IBM Journal of Research and Development, Vol. 8, No. 2, April, 1964, entitled "Instabilities of Current in III-V Semiconductors". Gunn noted that some semiconductors, such as Gallium Arsenide (Ga As), exhibit instabilities in their current-voltage characteristics above a voltage level known as the Gunn threshold. Subsequent work by Gunn and others has led to the development of a widely accepted theory of operation of Gunn effect devices, which are basically semiconductor diodes with cathode and anode terminals. The average carrier drift velocity in a Gunn diode first increases with increasing electric field, i.e., voltage, until the Gunn threshold is reached, after which the carrier drift velocity decreases, resulting in a negative differential conductivity above the threshold. A space charge builds up near the cathode of the device, and, above the threshold, is large enough to become a high electric field domain that reduces the electric field outside of the domain and also reduces the current through the device. Periodically, a domain breaks away from the cathode and drifts with the carrier stream to the anode of the device. Then the current increases again, until a new domain is formed, at which time the current abruptly falls. The time interval between current pulses thereby generated defines the fundamental frequency of oscillation of the device, which may be operated either as an oscillator or as an amplifier. A Gunn device is typically biased, by a direct current, to a point above the Gunn threshold, and it operating characteristics are determined in part by the impedance parameters of external circuit elements to which the device is connected.
Devices operating at extremely high frequencies, up to 300 gigahertz (GHz), require a waveguide structure to transmit the high-frequency energy away from the device. Typically, the waveguide structure has been fabricated separately from the semiconductor device itself, although it will be apparent that there are distinct advantages in combining a waveguide and high-frequency device, such as one of the Gunn type, in a single structure.
Prior to this invention, there have been a number of attempts to combine high-frequency semiconductor devices, such as Gunn devices, with various types of waveguides, and these will first be briefly discussed, to place the present invention in proper perspective. There are some devices that employ discrete Gunn-type components, as exemplified by those disclosed in U.S. Pat. No. 3,903,488 to Fong, and U.S. Pat. No. 3,986,153 to Kuno et al. The Fong patent relates to an improved form of a planar dielectric waveguide to which high-frequency devices such as Gunn diodes and IMPATT diodes (impact avalanche and transit time diodes) can be bonded. The Kuno patent similarly involves the use of discrete devices. So long as a Gunn type device or other active semiconductor device is in the discrete form, the well known advantages of monolithic fabrication are not realized, and the composite device will be relatively costly to produce and may not be totally reliable.
U.S. Pat. No. 3,975,690 to Fleming discloses a planar transmission line utilizing a semiconductor material of the type that exhibits the Gunn effect. The Fleming transmission line is both a Gunn type amplifier and a waveguide. Its use, however, is limited to relatively low-frequency applications, as determined by the width of a surface conductor strip on the device. Moreover, since heat is dissipated principally by conduction through a substrate on which the device is formed, its operation is limited to relatively low powers.
Another patent relating to the same general subject matter area is U.S. Pat. No. 3,866,142 to Jacobs et al, which discloses an integrated waveguide and passive high-frequency device, such as a PIN diode or mixer diode. Active devices, such as oscillators and amplifiers, are shown in the patent as being discrete from the waveguide structure.
It will be appreciated from the foregoing that there is still a need for an integrated structure that combines an active semiconductor device and a waveguide, capable of operation at extremely high frequencies and relatively high powers. The present invention fulfills this need.